The present invention is generally directed to an implantable cardiac device which is powered by a depletable power source. The present invention is more particularly directed to such a device which provides precise recommended replacement time and absolute replacement time (sometimes referred to as xe2x80x9cend-of-lifexe2x80x9d) indications based upon actual remaining energy capacity of the depletable power source.
Implantable cardiac devices are well known in the art. Such devices include implantable monitors which monitor heart activity over extended periods of time for diagnostic purposes, implantable pacemakers which both monitor heart activity and apply stimulation pulses to a heart when required to maintain normal sinus rhythm, and implantable cardioverter-defibrillators which monitor heart activity for tachyarrhythmias and apply shocking stimulation pulses to a heart to return the heart to normal sinus rhythm. Implantable cardioverter-defibrillators also commonly include pacemaker functionality.
Since implantable cardiac devices are implanted beneath the skin of a patient, they are powered by a depletable power source, such as a battery. When the remaining battery energy capacity falls below a certain lower limit corresponding to an absolute replacement time, sometimes referred to as xe2x80x9cend-of-lifexe2x80x9d (EOL), the device must be replaced. Further, prior to EOL, as for example 90 days prior to EOL, the battery will reach a remaining energy capacity corresponding to the recommended replacement time (RRT). An RRT indication is generally provided to alert the patient""s physician that EOL is imminent and is timed relative to EOL to afford the physician sufficient time to schedule replacement of the device before EOL is reached.
Most present RRT and EOL indicators utilize battery voltage and/or impedance and their correlation to the remaining battery capacity to provide information as to when RRT and EOL will be reached. Unfortunately, the tolerances in the battery voltage measurements as well as battery impedance have too wide a variance to provide an adequate time margin for replacement after an indication of RRT. Many implantable devices are thus explanted based upon these inadequate measurements. These indicators generally start providing remaining battery capacity information only during the final 20 percent of battery life because significant changes in these factors do not occur during the initial 80 percent of the battery life. This has resulted in conflicting RRT indications in the field.
Further, in some instances, follow-up times may be relatively long. This can result in an implantable cardiac device entering not only RRT but also even EOL without clinical assistance.
The present invention provides an implantable cardiac device and method which monitors actual remaining depletable power source energy capacity for making accurate RRT and/or EOL indications. The amount of current provided by the power source is constantly measured by a current monitor to derive a first average of current, provided by the power source since its initial hook-up or xe2x80x9cbeginning-of-lifexe2x80x9d (BOL), and a second average current, provided over a last 24 hour period.
The remaining energy capacity of the power source is determined by multiplying the first average current by the time since BOL to derive actual used capacity and then subtracting the actual used capacity from the initial capacity of the power source. An EOL date is then calculated by dividing the remaining energy capacity by the second current average to derive the remaining power source life and then adding the remaining power source life to a current date. The RRT date may then be determined by subtracting a fixed time period, such as 90 days, for example, from the EOL date.
The RRT and EOL dates provided are extremely accurate because they are based upon the actual remaining power source energy capacity. Further, a first alarm may be issued when the current date equals or exceeds the RRT date and a second alarm may be issued when the current date equals or exceeds the EOL date.